a) Field of the Invention
The present invention concerns an alloyed and dense anode sheet with local stress relaxation. More particularly, the invention concerns an electrochemical generator including a negative electrode comprising a sheet of a host metal such as aluminum, lead, silver, silicon, zinc, magnesium, carbon, or combinations thereof, the sheet of host metal being intended to later on constitute a negative electrode and having the property of locally absorbing any lateral expansion and of substantially preventing any change in the plane of the sheet during the formation of alloy between the host metal and the alkali metal, such as lithium.
b) Description of the Prior Art
The more technically advanced generators operating with polymer electrolytes utilize metallic lithium, some time sodium, or other alkali metals, as anode sheets. Alkali metals are malleable and may be used in the form of thin films (CA 2,099,526 and 2,099,524). However, in certain cases of extreme utilization, such as at temperatures higher than 100.degree. C., the use of metallic lithium or other alkali metals may cause the melting of lithium or the alkali metal and the destruction of the electrochemical cell. Moreover, under forced conditions of electrochemical cycling, the formation of dendrites for example of lithium may be induced, for example, when currents of recharge which are too elevated are used, with all the known disadvantages that this implies, while an alloy which operates at a more anodic potential, for example between +300 to 450 mV for lithium aluminum vs lithium, does not cause lithium deposit nor dendritic growth.
The use of alloys of alkali metals such as lithium has been proposed and demonstrated with success in the case of generators operating with molten salts media (U.S. Pat. No. 4,489,143). When operating with an organic medium, and more particularly with a polymer medium, where the thickness of the electrode films are lower than 100 micrometers (.mu.m) it becomes very difficult to operate with anodes in the form of sheets of alloys. Indeed, intermetallic compounds of lithium which can be used as anodes, such as LiAl, Li.sub.21 Si.sub.5, Li.sub.22 Pb.sub.5 and others are hard and brittle and cannot be laminated as is the case for lithium or weakly alloyed lithium.
It has been shown (CA 1,222,543) that these anodes may be prepared in the form of thin films by producing composites consisting of powders of the intermetallic compound bound by the polymer electrolyte, or still that it was possible under certain conditions to pre-alloy the sheet of host metal of the anode by chemically treating the surface of the sheet (U.S. Pat. No. 4,590,840) or by electrochemically loading part of this sheet (U.S. Pat. No. 4,794,060). However, these techniques which are operational under certain conditions utilize reactive materials, and the pre-inserted alloys are often pyrophoric or give rise to difficulties of operation and optimization of performances. When the anodes are prepared in discharged state, one of the major difficulties to overcome is due to the substantial volume variation resulting from the formation of the alloy which results in important stresses on the structure.
When it is intended to form the alloy from a sheet of host metal containing no lithium during or after assembling a polymer electrolyte generator, the expansion in volume of the structure in the direction of the thickness of the sheets may be compensated by a suitable design of the cell by providing for example for an increase of the total thickness of the superposed sheets, more especially because in the direction of thickness, variation is very small and therefore much more negligible.
The expansion of the host metal in the plane of the sheets is however accumulated along the entire surface of the latter and creates folds enabling to accommodate local stress generated by the expansion. The consequence is that short circuits are formed between the electrodes or that mechanical defects are produced, which harm the operation of a generator. This phenomenon is illustrated in the picture of FIG. 4b in the case of an ordinary sheet of aluminum which is alloyed with lithium in the polymer electrolyte device of FIG. 3. In a true generator, the developed surfaces, the adhesion of the films between one another and the pressure which is maintained on the entire cell prevent any sliding of the host structure to accommodate this lateral expansion.